In many communications networks there is a requirement for an accurate clock signal at each node of the network. The clock signal can be used to maintain synchronisation between the rate at which network nodes operate and to prevent problems such as the over-filling of network buffers.
In Synchronous Digital Hierarchy (SDH) networks, in which traffic flows are time-division multiplexed (TDM), accurate timing reference information is distributed to all SDH network nodes at the physical layer of the network. There is now considerable interest in distributing TDM traffic over packet-switched networks (PSN). Where TDM data is carried over a packet-switched network, and especially a packet-switched network comprising multiple independently-operated domains, the physical layer clock distribution techniques of an SDH network cannot be used. This poses a problem that the TDM network segments at each side of the packet-switched network no longer have ready access to the same clock reference.
Techniques for making a clock reference available at nodes of a packet-switched network include Differential Clock Recovery and Adaptive Clock Recovery (ACR). An overview of the Differential Clock Recovery method is shown in FIG. 1. An accurate reference clock 2 is made available at each network node such as by: providing each node with a GPS receiver which can derive an accurate GPS system clock; providing each node with an atomic reference; or distributing a common clock to each node. One technique for distributing a clock is the Precision Time Protocol (PTP) defined in IEEE 1588-2002. An ingress node to the PSN compares the clock rate of a signal 3 with the reference clock to form a clock differential, and sends the clock differential 4 to an egress node. The egress node can then use the received clock differential 4, and the reference clock 2, to recover a local clock 5 at the egress node. In an Adaptive Clock Recovery method a common clock is not made available at all network nodes. Instead, an egress node recovers a clock by using an averaging process 8 performed over a suitably long period of time. Where data is sent at a constant bit rate at the ingress side, the egress node knows that the average rate of the recovered data should equal the rate at which the data was sent. The quality of the recovered clock obtained using the Adaptive Clock Recovery method is affected by the Packet Delay Variation (PDV) in the PSN, and this can require a long averaging period at the egress node.
Both of the known methods have disadvantages. Differential Clock Recovery has a disadvantage of either requiring additional receiving apparatus at each node, or relies on a distribution path for the clock which can be different to that of the traffic. The Adaptive Clock Recovery method can be affected by synchronisation source switches (e.g. because of a protection switch in the synchronisation network) which can cause the clock source to move within the recommended standard requirements (e.g. G.823, G.813) but the recovered clock will not follow this variation because of the longer-term filtering used at the receive nodes.